The present invention relates to a motor drive system comprising a motor driven by a variable frequency power supply and a machine mechanically connected to and driven by the motor, and more particularly to a motor drive system which can effectively suppress, by a simple method, electric corrosion in the bearing portions of the drive shaft of the motor and the shaft of the driven machine.
Conventionally, a motor drive system of this type is arranged such that the drive shaft of the motor is directly connected to the rotation shaft of the driven machine. The motor is generally driven by a variable frequency power supply, in which the rotation can be easily controlled by means of an inverter. A voltage is supplied from the variable frequency power supply to a stator coil of the motor, so that the motor is driven.
The driven machine, such as a decelerator or a load machine, is mechanically coupled to the motor through the shafts of the driven machine and the motor. The shafts of the motor and the driven machine are supported by the frames thereof through the bearings.
The variable frequency power supply used as a power source of the motor obtains an AC driving voltage of 0 to 60 Hz from the direct current using a PWM (pulse width modulation) inverter or the like. In the aforementioned DC-AC conversion, a high-frequency voltage of 50 to 100 kHz is superposed on the AC driving voltage due to a steep change in voltage resulting from switching of the inverter circuit. When the AC driving voltage, on which the high-frequency voltage is superposed, is applied to the stator coil of the motor from the variable frequency power supply, a current, proportional to a ratio (dv/dt) of change in voltage supplied from the variable frequency power supply, flows through the frame of the motor via stray capacitance existing between the stator coil and the motor frame.
A pulse voltage or surge voltage superposed on the driving voltage fed from the variable frequency power supply flows as a current through the motor frame via the stray capacitance distributed between the stator coil and the motor frame. The current flows into an earth electrode (an electrode connected to an earth trunk line) via a motor earthing line connected to the motor frame. Then, the current is finally fed back to the variable frequency power supply via stray capacitance distributed between the variable frequency power supply and the earth electrode.
In this case, the current flowing through the stray capacitance between the stator coil and the frame is a resonant current produced by inductance of the stator coil, inductance between the variable frequency power supply and the stator coil, and the stray capacitance between the stator coil and the motor frame. In general, the frequency of the resonant current is as high as several tens of kHz to several MHz. The impedance between the motor frame and the earth electrode with respect to the high frequency current becomes high due to the skin effect of the conductor connecting the motor frame and the earth.
On the other hand, the frame of the driven machine is generally earthed to a base constituted by a steel frame of a building. The impedance between the frame of the driven machine and the earth electrode, in particular, the impedance with respect to a high frequency is low. Hence, the current flowing from the stator coil through the stray capacitance between the stator coil and the frame flows to the drive shaft of the motor via the bearings of the drive shaft. It further flows to the frame of the driven machine through the shaft of the driven machine connected to the drive shaft, and further to the earth electrode. Since the current flows through the bearings of the drive shaft of the motor and the bearings of the drive machine, it causes electric corrosion.
Conventionally, electric corrosion is prevented as follows. The motor frame and the drive shaft are short-circuited by an earth brush, so that the current flowing through the bearing of the drive shaft of the motor can be split.
In this method, however, the current flowing through the bearing of the drive shaft of the motor is reduced, whereas the current flowing through the bearings of the shaft of the driven machine is increased contrarily. For this reason, it is necessary to provide an earth brush between the shaft of the driven machine and the frame thereof. Further, since the earth brush is subject to wear, they must be exchanged periodically.